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Hypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobe Systems Incorporated345 Park AveSan JoseCA95110USAfielding@gbiv.comhttp://roy.gbiv.com/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/Applications
HTTPbis
The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for
distributed, collaborative, hypertext information systems. This document
defines HTTP/1.1 conditional requests, including metadata header fields
for indicating state changes, request header fields for making
preconditions on such state, and rules for constructing the responses to a
conditional request when one or more preconditions evaluate to false.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
Conditional requests are HTTP requests that include
one or more header fields indicating a precondition to be tested before
applying the method semantics to the target resource.
This document defines the HTTP/1.1 conditional request mechanisms in terms
of the architecture, syntax notation, and conformance criteria defined in
.
Conditional GET requests are the most efficient mechanism for HTTP
cache updates &caching;. Conditionals can also be
applied to state-changing methods, such as PUT and DELETE, to prevent
the "lost update" problem: one client accidentally overwriting
the work of another client that has been acting in parallel.
Conditional request preconditions are based on the state of the target
resource as a whole (its current value set) or the state as observed
in a previously obtained representation (one value in that set).
A resource might have multiple current representations, each with its
own observable state. The conditional request mechanisms assume that
the mapping of requests to a "selected representation" (&representation;)
will be consistent over time if the server intends to take advantage of
conditionals. Regardless, if the mapping is inconsistent and the server is
unable to select the appropriate representation, then no harm will result
when the precondition evaluates to false.
The conditional request preconditions defined by this specification
() are evaluated when applicable to the
recipient () according to their order of
precedence ().
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Conformance criteria and considerations regarding error handling
are defined in &conformance;.
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
with a list extension, defined in
&abnf-extension;, that allows for compact definition of
comma-separated lists using a '#' operator (similar to how the '*' operator
indicates repetition).
describes rules imported from
other documents.
shows the collected grammar with all list
operators expanded to standard ABNF notation.
This specification defines two forms of metadata that are commonly used
to observe resource state and test for preconditions: modification dates
() and opaque entity tags
(). Additional metadata that reflects resource state
has been defined by various extensions of HTTP, such as WebDAV
, that are beyond the scope of this specification.
A resource metadata value is referred to as a "validator"
when it is used within a precondition.
Validators come in two flavors: strong or weak. Weak validators are easy
to generate but are far less useful for comparisons. Strong validators
are ideal for comparisons but can be very difficult (and occasionally
impossible) to generate efficiently. Rather than impose that all forms
of resource adhere to the same strength of validator, HTTP exposes the
type of validator in use and imposes restrictions on when weak validators
can be used as preconditions.
A "strong validator" is representation metadata that changes value whenever
a change occurs to the representation data that would be observable in the
payload body of a 200 (OK) response to GET.
A strong validator might change for reasons other than a change to the
representation data, such as when a
semantically significant part of the representation metadata is changed
(e.g., Content-Type), but it is in the best interests of the
origin server to only change the value when it is necessary to invalidate
the stored responses held by remote caches and authoring tools.
Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an
entry using a validator that it obtained in the distant past.
A strong validator is unique across all versions of all
representations associated with a particular resource over time.
However, there is no implication of uniqueness across representations
of different resources (i.e., the same strong validator might be
in use for representations of multiple resources at the same time
and does not imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best are
based on strict revision control, wherein each change to a representation
always results in a unique node name and revision identifier being assigned
before the representation is made accessible to GET. A collision-resistant hash
function applied to the representation data is also sufficient if the data
is available prior to the response header fields being sent and the digest
does not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that differ
only in their metadata, such as might occur with content negotiation over
media types that happen to share the same data format, then the origin
server needs to incorporate additional information in the validator to
distinguish those representations.
In contrast, a "weak validator" is representation metadata that
might not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated, such
as clock resolution or an inability to ensure uniqueness for all possible
representations of the resource, or due to a desire by the resource owner
to group representations by some self-determined set of equivalency
rather than unique sequences of data. An origin server &SHOULD; change a
weak entity-tag whenever it considers prior representations to be
unacceptable as a substitute for the current representation. In other words,
a weak entity-tag ought to change whenever the origin server wants caches to
invalidate old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second and
retrieved between those modifications.
Likewise, a validator is weak if it is shared by two or more
representations of a given resource at the same time, unless those
representations have identical representation data. For example, if the
origin server sends the same validator for a representation with a gzip
content coding applied as it does for a representation with no content
coding, then that validator is weak. However, two simultaneous
representations might share the same strong validator if they differ only
in the representation metadata, such as when two different media types are
available for the same representation data.
Strong validators are usable for all conditional requests, including cache
validation, partial content ranges, and "lost update" avoidance.
Weak validators are only usable when the client does not require exact
equality with previously obtained representation data, such as when
validating a cache entry or limiting a web traversal to recent changes.
The "Last-Modified" header field in a response provides a timestamp
indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the conclusion
of handling the request.
Last-Modified = HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
An origin server &SHOULD; send Last-Modified for any selected
representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests
and evaluating cache freshness (&caching;) results in a substantial
reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be
the most recent time that any of those parts were changed.
How that value is determined for any given resource is an
implementation detail beyond the scope of this specification.
What matters to HTTP is how recipients of the Last-Modified
header field can use its value to make conditional requests
and test the validity of locally cached responses.
An origin server &SHOULD; obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to
make an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.
An origin server with a clock &MUST-NOT; send a Last-Modified date
that is later than the server's time of message origination (Date).
If the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server &MUST; replace that
value with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation.
An origin server without a clock &MUST-NOT; assign Last-Modified
values to a response unless these values were associated
with the resource by some other system or user with a reliable clock.
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
The validator is being compared by an origin server to the
actual current validator for the representation and,That origin server reliably knows that the associated representation did
not change twice during the second covered by the presented
validator.
or
The validator is about to be used by a client in an If-Modified-Since,
If-Unmodified-Since header field, because the client has
a cache entry, or If-Range for the associated
representation, andThat cache entry includes a Date value, which gives the
time when the origin server sent the original response, andThe presented Last-Modified time is at least 60 seconds before
the Date value.
or
The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, andThat cache entry includes a Date value, which gives the
time when the origin server sent the original response, andThe presented Last-Modified time is at least 60 seconds before
the Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The
arbitrary 60-second limit guards against the possibility that the Date and
Last-Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An
implementation &MAY; use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
The "ETag" header field in a response provides the current entity-tag for
the selected representation, as determined at the conclusion of handling
the request.
An entity-tag is an opaque validator for differentiating between
multiple representations of the same resource, regardless of whether
those multiple representations are due to resource state changes over
time, content negotiation resulting in multiple representations being
valid at the same time, or both. An entity-tag consists of an opaque
quoted string, possibly prefixed by a weakness indicator.
ETag = entity-tagentity-tag = [ weak ] opaque-tagweak = "W/" ; "W/", case-sensitive
opaque-tag = DQUOTE *etagcDQUOTEetagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
&Note; Previously, opaque-tag was defined to be a quoted-string
(), thus some recipients
might perform backslash unescaping. Servers therefore ought to avoid
backslash characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently maintained.
Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity-tag can be either a weak or strong validator, with
strong being the default. If an origin server provides an entity-tag
for a representation and the generation of that entity-tag does not satisfy
all of the characteristics of a strong validator
(), then the origin server
&MUST; mark the entity-tag as weak by prefixing its opaque value
with "W/" (case-sensitive).
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the
most accurate and efficient validation mechanism for that resource,
and that any such mechanism can be mapped to a simple sequence of
octets for easy comparison. Since the value is opaque, there is no
need for the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations.
Other implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution.
An origin server &SHOULD; send ETag for any selected representation
for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and
evaluating cache freshness (&caching;) can result in a substantial
reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability.
There are two entity-tag comparison functions, depending
on whether the comparison context allows the use of weak validators
or not:
Strong comparison: two entity-tags are equivalent if both
are not weak and their opaque-tags match character-by-character.Weak comparison: two entity-tags are equivalent if their opaque-tags
match character-by-character, regardless of either or both
being tagged as "weak".
The example below shows the results for a set of entity-tag pairs,
and both the weak and strong comparison function results:
ETag 1ETag 2Strong ComparisonWeak ComparisonW/"1"W/"1"no matchmatchW/"1"W/"2"no matchno matchW/"1""1"no matchmatch"1""1"matchmatch
Consider a resource that is subject to content negotiation
(&content-negotiation;), and where the representations sent in response to
a GET request vary based on the Accept-Encoding request
header field (&header-accept-encoding;):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content coding.
If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length:
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would be:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
&Note; Content codings are a property of the representation data,
so a strong entity-tag for a content-encoded representation has to be
distinct from the entity tag of an unencoded representation to prevent
potential conflicts during cache updates and range requests. In contrast,
transfer codings (&transfer-codings;) apply only during message transfer
and do not result in distinct entity-tags.
In 200 (OK) responses to GET or HEAD, an origin server:
&SHOULD; send an entity-tag validator unless it is not feasible to
generate one.&MAY; send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags,
or if it is unfeasible to send a strong entity-tag.&SHOULD; send a Last-Modified value if it is feasible to
send one.
In other words, the preferred behavior for an origin server
is to send both a strong entity-tag and a Last-Modified
value in successful responses to a retrieval request.
A client:
&MUST; send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an
entity-tag has been provided by the origin server.&SHOULD; send the Last-Modified value in non-subrange
cache validation requests (using If-Modified-Since)
if only a Last-Modified value has been provided by the origin server.&MAY; send the Last-Modified value in subrange
cache validation requests (using If-Unmodified-Since)
if only a Last-Modified value has been provided by an HTTP/1.0 origin
server. The user agent &SHOULD; provide a way to disable this, in case
of difficulty.&SHOULD; send both validators in cache validation requests if both an
entity-tag and a Last-Modified value have been provided
by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
This section defines the syntax and semantics of HTTP/1.1 header fields
for applying preconditions on requests.
defines when the preconditions are applied.
defines the order of evaluation when more than
one precondition is present.
The "If-Match" header field makes the request method conditional on the
recipient origin server either having at least one current
representation of the target resource, when the field-value is "*", or
having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in the
field-value.
An origin server &MUST; use the strong comparison function when comparing
entity-tags for If-Match (), since
the client intends this precondition to prevent the method from being
applied if there have been any changes to the representation data.
If-Match = "*" / 1#entity-tag
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
If-Match is most often used with state-changing methods (e.g., POST, PUT,
DELETE) to prevent accidental overwrites when multiple user agents might be
acting in parallel on the same resource (i.e., to prevent the "lost update"
problem). It can also be used with safe methods to abort a request if the
selected representation does not match one already stored
(or partially stored) from a prior request.
An origin server that receives an If-Match header field &MUST; evaluate the
condition prior to performing the method ().
If the field-value is "*", the condition is false if the origin server
does not have a current representation for the target resource.
If the field-value is a list of entity-tags, the condition is false if
none of the listed tags match the entity-tag of the selected representation.
An origin server &MUST-NOT; perform the requested method if a received
If-Match condition evaluates to false; instead the origin server &MUST;
respond with either:
a) the 412 (Precondition Failed) status code; or,
b) one of the 2xx (Successful) status codes if the origin
server has verified that a state change is being requested and the final
state is already reflected in the current state of the target resource
(i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of it, perhaps because the prior response
was lost or a compatible change was made by some other user agent).
In the latter case, the origin server &MUST-NOT; send a validator header
field in the response unless it can verify that the request is a duplicate
of an immediately prior change made by the same user agent.
The If-Match header field can be ignored by caches and intermediaries
because it is not applicable to a stored response.
The "If-None-Match" header field makes the request method conditional on
a recipient cache or origin server either not having any current
representation of the target resource, when the field-value is "*", or
having a selected representation with an entity-tag that does not match any
of those listed in the field-value.
A recipient &MUST; use the weak comparison function when comparing
entity-tags for If-None-Match (),
since weak entity-tags can be used for cache validation even if there have
been changes to the representation data.
If-None-Match = "*" / 1#entity-tag
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. When a client desires to update one or more stored
responses that have entity-tags, the client &SHOULD; generate an
If-None-Match header field containing a list of those entity-tags when
making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those
stored responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an unsafe
request method (e.g., PUT) from inadvertently modifying an existing
representation of the target resource when the client believes that
the resource does not have a current representation (&safe-methods;).
This is a variation on the "lost update" problem that might arise if more
than one client attempts to create an initial representation for the target
resource.
An origin server that receives an If-None-Match header field &MUST;
evaluate the condition prior to performing the method
().
If the field-value is "*", the condition is false if the origin server
has a current representation for the target resource.
If the field-value is a list of entity-tags, the condition is false if
one of the listed tags match the entity-tag of the selected representation.
An origin server &MUST-NOT; perform the requested method if the condition
evaluates to false; instead, the origin server &MUST; respond with either
a) the 304 (Not Modified) status code if the request method
is GET or HEAD; or,
b) the 412 (Precondition Failed) status code for all other
request methods.
Requirements on cache handling of a received If-None-Match header field
are defined in &cache-validation-received;.
The "If-Modified-Since" header field makes a GET or HEAD request method
conditional on the selected representation's modification date being more
recent than the date provided in the field-value. Transfer of the selected
representation's data is avoided if that data has not changed.
If-Modified-Since = HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient &MUST; ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in
If-None-Match is considered to be a more accurate
replacement for the condition in If-Modified-Since and the two are only
combined for the sake of interoperating with older intermediaries that
might not implement If-None-Match.
A recipient &MUST; ignore the If-Modified-Since header field if the
received field-value is not a valid HTTP-date, or if the request method
is neither GET nor HEAD.
A recipient &MUST; interpret an If-Modified-Since field-value's timestamp
in terms of the origin server's clock.
If-Modified-Since is typically used for two distinct purposes:
1) to allow efficient updates of a cached representation that does not
have an entity-tag; and,
2) to limit the scope of a web traversal to resources that have recently
changed.
When used for cache updates, a cache will typically use the value of the
cached message's Last-Modified field to generate the field
value of If-Modified-Since. This behavior is most interoperable for cases
where clocks are poorly synchronized or when the server has chosen to only
honor exact timestamp matches (due to a problem with Last-Modified dates
that appear to go "back in time" when the origin server's clock is
corrected or a representation is restored from an archived backup).
However, caches occasionally generate the field value based on other data,
such as the Date header field of the cached message or the
local clock time that the message was received, particularly when the
cached message does not contain a Last-Modified field.
When used for limiting the scope of retrieval to a recent time window, a
user agent will generate an If-Modified-Since field value based on either
its own local clock or a Date header field received from the
server in a prior response. Origin servers that choose an exact timestamp
match based on the selected representation's Last-Modified
field will not be able to help the user agent limit its data transfers to
only those changed during the specified window.
An origin server that receives an If-Modified-Since header field &SHOULD;
evaluate the condition prior to performing the method
().
The origin server &SHOULD-NOT; perform the requested method if the selected
representation's last modification date is earlier than or equal to the
date provided in the field-value; instead, the origin server &SHOULD;
generate a 304 (Not Modified) response, including only those
metadata that are useful for identifying or updating a previously cached
response.
Requirements on cache handling of a received If-Modified-Since header field
are defined in &cache-validation-received;.
The "If-Unmodified-Since" header field makes the request method conditional
on the selected representation's last modification date being earlier than or
equal to the date provided in the field-value. This field accomplishes the
same purpose as If-Match for cases where the user agent does
not have an entity-tag for the representation.
If-Unmodified-Since = HTTP-date
An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient &MUST; ignore If-Unmodified-Since if the request contains an
If-Match header field; the condition in
If-Match is considered to be a more accurate replacement for
the condition in If-Unmodified-Since and the two are only combined for the
sake of interoperating with older intermediaries that might not implement
If-Match.
A recipient &MUST; ignore the If-Unmodified-Since header field if the
received field-value is not a valid HTTP-date.
A recipient &MUST; interpret an If-Unmodified-Since field-value's timestamp
in terms of the origin server's clock.
If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when multiple
user agents might be acting in parallel on a resource that does
not supply entity-tags with its representations (i.e., to prevent the
"lost update" problem). It can also be used with safe methods to abort a
request if the selected representation does not match one
already stored (or partially stored) from a prior request.
An origin server that receives an If-Unmodified-Since header field &MUST;
evaluate the condition prior to performing the method
().
The origin server &MUST-NOT; perform the requested method if the selected
representation's last modification date is more recent than the date
provided in the field-value; instead the
origin server &MUST; respond with either:
a) the 412 (Precondition Failed) status code; or,
b) one of the 2xx (Successful) status codes if the origin
server has verified that a state change is being requested and the final
state is already reflected in the current state of the target resource
(i.e., the change requested by the user agent has already succeeded, but
the user agent might not be aware of that because the prior response message
was lost or a compatible change was made by some other user agent).
In the latter case, the origin server &MUST-NOT; send a validator header
field in the response unless it can verify that the request is a duplicate
of an immediately prior change made by the same user agent.
The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response.
The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and
If-Unmodified-Since header fields but instructs the
recipient to ignore the Range header field if the validator
doesn't match, resulting in transfer of the new selected representation
instead of a 412 response. If-Range is defined in &header-if-range;.
The 304 (Not Modified) status code indicates that a
conditional GET or HEAD request has been
received and would have resulted in a 200 (OK) response
if it were not for the fact that the condition has evaluated to false.
In other words, there is no need for the server to transfer a
representation of the target resource because the request indicates that
the client, which made the request conditional, already has a valid
representation; the server is therefore redirecting the client to make
use of that stored representation as if it were the payload of a
200 (OK) response.
The server generating a 304 response &MUST; generate any of the following
header fields that would have been sent in a 200 (OK)
response to the same request:
Cache-Control,
Content-Location,
Date,
ETag,
Expires, and
Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations,
a sender &SHOULD-NOT; generate representation metadata other
than the above listed fields unless said metadata exists for the
purpose of guiding cache updates (e.g., Last-Modified might
be useful if the response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in
&freshening-responses;. If the conditional request originated with an
outbound client, such as a user agent with its own cache sending a
conditional GET to a shared proxy, then the proxy &SHOULD; forward the
304 response to that client.
A 304 response cannot contain a message-body; it is always
terminated by the first empty line after the header fields.
The 412 (Precondition Failed) status code indicates that one
or more conditions given in the request header fields evaluated to false
when tested on the server. This response code allows the client to place
preconditions on the current resource state (its current representations
and metadata) and thus prevent the request method from being applied if the
target resource is in an unexpected state.
Except when excluded below, a recipient cache or origin server &MUST;
evaluate received request preconditions after it has successfully performed
its normal request checks and just before it would perform the action
associated with the request method.
A server &MUST; ignore all received preconditions if its response to the
same request without those conditions would have been a status code other
than a 2xx or 412 (Precondition Failed).
In other words, redirects and failures take precedence over the evaluation
of preconditions in conditional requests.
A server that is not the origin server for the target resource and cannot
act as a cache for requests on the target resource &MUST-NOT; evaluate the
conditional request header fields defined by this specification, and
&MUST; forward them if the request is forwarded, since the generating
client intends that they be evaluated by a server that can provide a
current representation.
Likewise, a server &MUST; ignore the conditional request header fields
defined by this specification when received with a request method that does
not involve the selection or modification of a
selected representation, such as CONNECT, OPTIONS, or TRACE.
Conditional request header fields that are defined by extensions to HTTP
might place conditions on all recipients, on the state of the target
resource in general, or on a group of resources. For instance, the "If"
header field in WebDAV can make a request conditional on various aspects
of multiple resources, such as locks, if the recipient understands and
implements that field ().
Although conditional request header fields are defined as being usable with
the HEAD method (to keep HEAD's semantics consistent with those of GET),
there is no point in sending a conditional HEAD because a successful
response is around the same size as a 304 (Not Modified)
response and more useful than a 412 (Precondition Failed)
response.
When more than one conditional request header field is present in a request,
the order in which the fields are evaluated becomes important. In practice,
the fields defined in this document are consistently implemented in a
single, logical order, since "lost update" preconditions have more strict
requirements than cache validation, a validated cache is more efficient
than a partial response, and entity tags are presumed to be more accurate
than date validators.
A recipient cache or origin server &MUST; evaluate the request
preconditions defined by this specification in the following order:
When recipient is the origin server and
If-Match is present,
evaluate the If-Match precondition:
if true, continue to step if false, respond 412 (Precondition Failed) unless
it can be determined that the state-changing request has already
succeeded (see )When recipient is the origin server,
If-Match is not present, and
If-Unmodified-Since is present,
evaluate the If-Unmodified-Since precondition:
if true, continue to step if false, respond 412 (Precondition Failed) unless
it can be determined that the state-changing request has already
succeeded (see )When If-None-Match is present,
evaluate the If-None-Match precondition:
if true, continue to step if false for GET/HEAD, respond 304 (Not Modified)if false for other methods, respond 412 (Precondition Failed)When the method is GET or HEAD,
If-None-Match is not present, and
If-Modified-Since is present,
evaluate the If-Modified-Since precondition:
if true, continue to step if false, respond 304 (Not Modified)When the method is GET and both
Range and If-Range are present,
evaluate the If-Range precondition:
if the validator matches and the Range specification is
applicable to the selected representation, respond
206 (Partial Content)Otherwise,
all conditions are met, so perform the requested action and
respond according to its success or failure.
Any extension to HTTP/1.1 that defines additional conditional request
header fields ought to define its own expectations regarding the order
for evaluating such fields in relation to those defined in this document
and other conditionals that might be found in practice.
The HTTP Status Code Registry located at
shall be updated with the registrations below:
ValueDescriptionReference304Not Modified412Precondition Failed
HTTP header fields are registered within the Message Header Field Registry
maintained at
.
This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the permanent
registrations below (see ):
Header Field NameProtocolStatusReferenceETaghttpstandardIf-MatchhttpstandardIf-Modified-SincehttpstandardIf-None-MatchhttpstandardIf-Unmodified-SincehttpstandardLast-Modifiedhttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
This section is meant to inform developers, information providers, and
users of known security concerns specific to the HTTP conditional
request mechanisms. More general security considerations are addressed
in HTTP messaging &messaging; and semantics &semantics;.
The validators defined by this specification are not intended to ensure
the validity of a representation, guard against malicious changes, or
detect man-in-the-middle attacks. At best, they enable more efficient cache
updates and optimistic concurrent writes when all participants are behaving
nicely. At worst, the conditions will fail and the client will receive a
response that is no more harmful than an HTTP exchange without conditional
requests.
An entity-tag can be abused in ways that create privacy risks. For example,
a site might deliberately construct a semantically invalid entity-tag that
is unique to the user or user agent, send it in a cacheable response with a
long freshness time, and then read that entity-tag in later conditional
requests as a means of re-identifying that user or user agent. Such an
identifying tag would become a persistent identifier for as long as the
user agent retained the original cache entry. User agents that cache
representations ought to ensure that the cache is cleared or replaced
whenever the user performs privacy-maintaining actions, such as clearing
stored cookies or changing to a private browsing mode.
See &acks;.
Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deHypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.de2xx2xx (Successful)200 (OK)204 (No Content)Accept-EncodingContent-LocationContent-TypeDateLocationVaryselected representationHypertext Transfer Protocol (HTTP/1.1): Range RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deIf-RangeRange206 (Partial Content)Hypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporatedfielding@gbiv.comAkamaimnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deCache-ControlExpiresKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgHTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)CommerceNetldusseault@commerce.net
The definition of validator weakness has been expanded and clarified.
()
Weak entity-tags are now allowed in all requests except range requests.
(Sections and
)
The ETag header field ABNF has been changed to not use
quoted-string, thus avoiding escaping issues.
()
ETag is defined to provide an entity tag for the selected representation,
thereby clarifying what it applies to in various situations (such as a
PUT response).
()
The precedence for evaluation of conditional requests has been defined.
()
The following core rules are included by
reference, as defined in :
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character).
The rules below are defined in :
OWS = <OWS, defined in &whitespace;>
obs-text = <obs-text, defined in &field-components;>
The rules below are defined in other parts:
HTTP-date = <HTTP-date, defined in &http-date;>
In the collected ABNF below, list rules are expanded as per .
ETag = entity-tag
HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1>
If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Modified-Since = HTTP-date
If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Unmodified-Since = HTTP-date
Last-Modified = HTTP-date
OWS = <OWS, defined in [Part1], Section 3.2.3>
entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text
obs-text = <obs-text, defined in [Part1], Section 3.2.6>
opaque-tag = DQUOTE *etagc DQUOTE
weak = %x57.2F ; W/
Changes up to the IETF Last Call draft are summarized
in .
Closed issues:
:
"APPSDIR review of draft-ietf-httpbis-p4-conditional-24"
Closed issues:
:
"add 'stateless' to Abstract"
:
"improve introduction of list rule"
:
"augment security considerations with pointers to current research"